The present invention relates to a laser diode driving apparatus for driving multiple laser diodes in an optical disc recording/reproducing apparatus or the like.
In recent years, recording media for use in optical disc recording/reproducing apparatuses are getting diversified depending on usage. In addition to CD and DVD, BD (Blu-ray Disc) and HD-DVD (High-Definition DVD) are being used increasingly. For the purpose of dealing with various kinds of recording media, it is necessary to use laser diodes for outputting light having wavelengths suited for respective recording media. Hence, multiple laser diodes are mounted in an optical disc recording/reproducing apparatus capable of dealing with multiple kinds of recording media. Generally, multiple laser diodes are driven by a single laser diode driving apparatus.
In the reproduction mode of a conventional optical disc recording/reproducing apparatus, there is a problem of the so-called optical feedback noise. The optical feedback noise occurs when the light applied from a laser diode to a recording medium in the reproduction mode is reflected by the recording medium and part of the reflected light returns to the laser diode, thereby hindering stable oscillation of the laser diode. Generally, a high-frequency signal having a frequency of several hundred MHz is superimposed in the reproduction mode, and the laser diode is driven, whereby the laser diode usually being oscillated in a single mode is oscillated in a multi mode, thereby solving the problem of the optical feedback noise.
In addition, the magnitude of the amplitude of the high-frequency superimposed current that is used in the reproduction mode changes significantly depending on the junction capacitance and the differential resistance of the laser diode to be driven, and the characteristics of other optical pickup components, such as the impedance of the transmission path from the laser diode driving apparatus to the laser diode. Furthermore, the frequency of the high-frequency superimposed current that is used in the reproduction mode is generally several hundred MHz. The higher harmonics of the high-frequency superimposed current may jeopardize EMI (electromagnetic interface), that is a standard of the electromagnetic interference. Conventionally, the amplitude of the high-frequency superimposed current is set using an external amplitude adjusting resistor. Japanese Patent Application Laid-open Publication No. 2004-342657 is taken as a conventional example in which such an external amplitude adjusting resistor is disclosed. FIG. 11 is a characteristic graph showing the relationship between the amplitude [mApp] of the high-frequency superimposed current described above and the resistance value [Ω] of the amplitude adjusting resistor. As shown in the characteristic graph of FIG. 11, it can be understood the amplitude of the high-frequency superimposed current decreases (I1→I2→I3) as the resistance value of the amplitude adjusting resistor is changed (R1→R2→R3).
However, as described above, the setting of the high-frequency superimposed current changes significantly depending on the junction capacitance and the differential resistance of the laser diode to be driven, and the characteristics of other optical pickup components, such as the impedance of the transmission path from the laser diode driving apparatus to the laser diode. In the method for setting the amplitude of the high-frequency superimposed current using the external amplitude adjusting resistor (Japanese Patent Application Laid-open Publication No. 2004-342657), there is a problem that the number of external components increases as the number of laser diodes increases.
The characteristics of laser diodes being used for optical disc recording/reproducing apparatuses have large variations among manufacturers. Japanese Patent Application Laid-open Publication No. 2004-288842 and Japanese Patent Application Laid-open Publication No. 2004-110975 disclose apparatuses in which the amplitude of the high-frequency superimposed current is set using an adjusting circuit incorporated therein. However, when the amplitude of the high-frequency superimposed current is set using the adjusting circuit incorporated inside the apparatus, there is a problem that the range of changing the setting is limited in comparison with the change in the characteristics of the laser diode.